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Homework Help: Vector Sum (Angle Theta and Magnitude)

  1. Jul 24, 2011 #1
    1. The problem statement, all variables and given/known data




    I made those illustrations up. :) We are asked to find the angle theta using the graphical method (Tail-to-Tip Method)
    From these examples, Vector A + Vector B + Vector C = Resultant Vector

    How to find angle theta of Resultant Vector?

    2. Relevant equations

    3. The attempt at a solution

    So, should I measure the angle of the resultant vector from the positive x-axis or is my understanding wrong? All I can find on the net is algebraic method and I can't even understand this graphical method yet.

    So, from illustration number 1, should it be somewhere close to 90 to 110 degrees?
    I don't even have a clue on how to know the angle theta of illustrations 2 and 3.

    I know using the algebraic method is much accurate and useful, but our teacher hasn't even taught us that yet. So please, someone help me? :)
  2. jcsd
  3. Jul 24, 2011 #2


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    Gold Member

    Well, what were the magnitudes/directions of the initial vectors? How did you decide what direction they pointed/how long they were?

    If you did indeed draw the vectors perfectly, then you can get not-super-accurate value for the resulting vector just by measuring it's length and direction.
  4. Jul 24, 2011 #3
    Well, I didn't mind putting the values since I'd still be doing the measuring using the protractor by myself. I know how to measure the magnitude, I'm just asking how / what is the proper procedure in measuring angle theta using a protractor as described in the graphical method.

    But if it helps here it is:

    Illustration 1:
    Vector A= Southwest, 8 cm
    Vector B= East, 6 cm

    Illustration 2:
    Vector A= West, 4 cm
    Vector B= 20 Degrees South of East , 5 cm
    Vector C= 57 Degrees South of West, 8 cm

    Illustration 3:
    Vector A= SouthWest, 7 cm
    Vector B= East, 4 cm

    Hope you can help me with this. :) And sorry if it is so inaccurate, I did try my best to make it look like my drawing in my paper. :)
  5. Jul 24, 2011 #4


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    Gold Member

    Have you done trigonometry? I am inclined to believe you have.
    Surely you can find the unknown angle pretty easily?

    You needn't draw the sketch perfectly, the important thing is the values with which you do the maths.
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